U.S. patent number 6,127,663 [Application Number 09/169,362] was granted by the patent office on 2000-10-03 for electronics cabinet cooling system.
This patent grant is currently assigned to Ericsson Inc.. Invention is credited to Clifford T. Jones.
United States Patent |
6,127,663 |
Jones |
October 3, 2000 |
Electronics cabinet cooling system
Abstract
A cabinet for housing telecommunications equipment in outdoor
locations includes an enclosure for containing the
telecommunications equipment. A subrack is mounted within the
enclosure. A duct directs ambient air from outside the cabinet to
the subrack. The duct includes an inlet disposed in a lower portion
of the cabinet and a subrack inlet plenum disposed immediately
below the subrack. A subrack exit plenum is disposed above the
electronic subrack for exhausting air heated by the electronics
equipment. A fan draws ambient air into the inlet through the duct
into the subrack inlet plenum, through the electronic subrack into
the subrack exit plenum, and out of the enclosure through the
subrack exhaust plenum. A heater is provided for heating inlet air
when ambient temperatures are low. The fan and heater are
controlled by a thermal controller. At low ambient temperatures,
the air flow is maintained constant by the fans and power to the
heater is varied to maintain the subrack inlet temperature
constant. At moderate temperatures, the heating element is turned
off and the fan speed is varied up to the maximum fan speed. At
high temperatures, the fans operate at full speed to maintain
maximum air flow.
Inventors: |
Jones; Clifford T. (Raleigh,
NC) |
Assignee: |
Ericsson Inc. (Research
Triangle Park, NC)
|
Family
ID: |
22615351 |
Appl.
No.: |
09/169,362 |
Filed: |
October 9, 1998 |
Current U.S.
Class: |
219/553; 219/209;
361/724 |
Current CPC
Class: |
F26B
21/06 (20130101); H05K 7/20572 (20130101); H05K
7/207 (20130101) |
Current International
Class: |
F26B
21/06 (20060101); H05K 7/20 (20060101); H05B
003/10 () |
Field of
Search: |
;219/520,521,524,530,538,540,552,553,385,386,209,210,444.1,400,491
;361/724,725,726,727 ;312/236 ;392/379,383 ;34/76,77 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Walberg; Teresa
Assistant Examiner: Dahbour; Fadi H.
Attorney, Agent or Firm: Coats & Bennett, PLLC
Claims
What is claimed is:
1. A cabinet for housing operative telecommunications equipment in
outdoor locations comprising:
a) an enclosure for containing said operative telecommunications
equipment, said enclosure adapted to be located outdoors;
b) an electronics subrack contained within said enclosure and
adapted to hold one or more circuit modules during operation of
said telecommunications equipment;
c) a duct for directing ambient air from outside said cabinet to
said subrack, said duct including an inlet disposed adjacent a
lower end of said cabinet and a subrack inlet plenum disposed
adjacent to an underside of said electronics subrack, wherein said
duct includes a heating chamber between said inlet and said subrack
inlet plenum;
d) a subrack exit plenum including an outlet disposed adjacent an
upper side of said electronics subrack; and
e) at least one fan for drawing ambient air into said inlet,
through said duct into said subrack inlet plenum, through said
electronics subrack into said subrack exit plenum, and through said
outlet out of said enclosure.
2. The cabinet of claim 1 further including a heating element
disposed within said heating chamber for heating ambient air drawn
through said inlet.
3. The cabinet of claim 2 wherein said heating element is an
electric resistance heater.
4. The cabinet of claim 2 further including a thermal controller
for controlling power to said heating element.
5. The cabinet of claim 4 wherein, in a low temperature mode, said
thermal controller maintains air flow constant and varies power to
said heating element.
6. The cabinet of claim 5 wherein, in a moderate temperature mode,
said thermal controller varies said air flow and shuts off power to
said heating element.
7. The cabinet of claim 6 wherein said air flow is varied
non-linearly in said moderate temperature mode.
8. The cabinet of claim 6 wherein said air flow is varied in said
moderate temperature mode so as to maintain outlet temperature
constant within the operational limits of the fan.
9. The cabinet of claim 6 wherein, in a high temperature mode, the
thermal controller shuts off power to said heating element and
operates said fan at maximum capacity.
10. The cabinet of claim 4 wherein said thermal controller operates
said heating element and said fan to maintain the rate of change of
temperature in said subrack inlet plenum to less than 0.5.degree.
C. per minute.
11. A cabinet for housing telecommunications equipment in outdoor
locations comprising:
a) an enclosure for containing said telecommunications
equipment;
b) an electronics subrack contained within said enclosure;
c) an inlet opening disposed in a lower end of said cabinet below
said subrack to allow ambient air to enter said enclosure;
d) an outlet opening disposed in an upper portion of said cabinet
above said subrack to expel air from said cabinet;
e) at least one fan for drawing ambient air into said inlet,
through said subrack and out of said outlet opening;
f) a heating element for heating ambient air drawn in through said
inlet before the air reaches said subrack;
g) a thermal controller for controlling the operation of said
heating element and said fan to maintain the temperature within
said enclosure within a predetermined thermal envelope;
h) wherein said thermal controller is operative to maintain air
flow constant and vary power to said heating element at ambient
temperatures in a low temperature mode, and shut off power to said
heating element and to vary said air flow in a moderate temperature
mode.
12. The cabinet of claim 11 wherein said air flow is varied
non-linearly over a range of temperatures in said moderate
temperature mode.
13. The cabinet of claim 11 wherein said airflow is varied in said
moderate temperature mode so as to maintain outlet temperature
constant within the operational limits of the fan.
14. The cabinet of claim 11 wherein said subrack having an inlet
plenum and said thermal controller operates said heating element
and said fan to maintain the rate of change of temperature in said
subrack inlet plenum to less than 0.5.degree. C. per minute.
15. The cabinet of claim 11 further including a duct within said
cabinet to direct air from said inlet opening to said subrack.
16. The cabinet of claim 15 wherein said duct includes a heating
chamber and wherein said heating element is disposed in said
heating chamber.
17. The cabinet of claim 11 wherein said heating element is an
electrical resistance heater.
18. A cabinet for housing operative telecommunications equipment in
outdoor locations comprising:
an enclosure for containing said operative telecommunications
equipment, said enclosure adapted to be positioned outdoors;
an electronics subrack contained within said enclosure and adapted
to hold one or more circuit modules during operation of said
telecommunications equipment;
a duct for directing ambient air from outside said cabinet to said
subrack, said duct extending through said enclosure from an air
inlet disposed adjacent a lower end of said cabinet and a subrack
inlet plenum disposed adjacent an underside of said electronics
subrack, and defining a segregated air flow path within said
enclosure;
a heating chamber in said duct between said air inlet and said
subrack inlet plenum;
an exhaust extending from said subrack to an outlet in said
enclosure; and
at least one fan for drawing ambient air into said inlet, through
said duct to said subrack, and exhausting air through said outlet
in said enclosure.
19. The cabinet of claim 18 further including a heating element
disposed within said heating chamber for heating ambient air drawn
in through said inlet.
20. The cabinet of claim 19 wherein said heating element is an
electric resistance heater.
21. The cabinet of claim 19 further including a thermal controller
for controlling power to said heating element.
22. The cabinet of claim 21 wherein, in a low temperature mode,
said thermal controller maintains air flow constant and varies
power to said heating element.
23. The cabinet of claim 21, wherein, in a moderate temperature
mode, said thermal controller varies air flow and shuts off power
to said heating element.
24. The cabinet of claim 23 wherein said air flow is varied
non-linearly in said moderate temperature mode.
25. The cabinet of claim 23 wherein said air flow is varied in said
moderate temperature mode so as to maintain outlet temperature
constant within the operational limits of the fan.
26. The cabinet of claim 23 wherein, in a high temperature mode,
the thermal controller shuts off power to said heating element and
operates said fan at maximum capacity.
27. The cabinet of claim 21 wherein said thermal controller
operates said heating element and said fan to maintain the rate of
change of temperature in said subrack inlet plenum to less than
0.5.degree. C. per minute.
28. The cabinet of claim 18 wherein said cabinet further includes
an equipment space disposed within said cabinet between said inlet
and said subrack, and wherein said duct extends through said
equipment space.
29. The cabinet of claim 28 further including power electronics
contained within said equipment space.
30. A cabinet for housing operative telecommunications equipment in
outdoor locations comprising:
an enclosure adapted to be positioned outdoors for containing said
operative telecommunications equipment, said enclosure having an
inlet and an outlet and defining an enclosed equipment space;
an electronics subrack contained within said enclosure and adapted
to hold one or more circuit modules during operation of said
telecommunications equipment, said electronics subrack dividing
said equipment space;
a duct disposed within said enclosure for directing ambient air
from outside said cabinet to said subrack, said duct extending
through said equipment space from said inlet to said electronics
subrack and defining a segregated air flow path within said
equipment space;
an exhaust extending from said subrack to an outlet in said
enclosure; and
at least one fan for drawing ambient air into said inlet, through
said duct to said subrack, and exhausting air through said outlet
in said enclosure.
Description
FIELD OF THE INVENTION
The present invention relates generally to cabinets for electronic
equipment and, more particularly, to a telecommunications cabinet
for housing telecommunications equipment in outdoor locations.
BACKGROUND OF THE INVENTION
Telecommunications equipment is typically designed for indoor
temperature controlled environments, but is often deployed in
outdoor locations. When telecommunications equipment is deployed in
outdoor locations, a cabinet is typically used to provide
environmental protection and control thermal conditions within the
design limits of the telecommunications equipment.
There are essentially two main approaches to housing
telecommunications equipment in outdoor cabinets--sealed cabinets
and ventilated cabinets. Ventilated cabinets use natural or forced
convection to draw ambient air through the cabinet to cool the
equipment inside. Natural convection is only feasible for low
density heat dissipation. At moderate and high power densities,
forced ventilation is required.
Conventional cabinet ventilation systems use one or more fans to
draw enough air into the cabinet to maintain the temperature below
maximum equipment limits and to return the heated air to the
outside environment. At low temperatures, the fans are turned off
by a thermostat. In extremely cold climates, an electric heater is
used to maintain the cabinet temperature within minimum equipment
temperatures.
A drawback to the use of ventilated cabinets is that the control of
the fans and heater is normally limited to on/off operation. This
method of operation can lead to abrupt changes in temperature and
create hot spots in the cabinet. In some cases, the equipment
specifications limit the allowed rate of change in cooling air
temperature to between 0.5 and 1.0.degree. C. per minute.
Ordinarily, this rate of change cannot be guaranteed with
conventional ventilated cabinets.
Sealed cabinets provide an alternative to ventilated cabinets for
housing telecommunications equipment in outdoor locations. Sealed
cabinets provide maximum protection from airborne contaminants, but
require special attention to the task of removing heat dissipated
by the equipment. A variety of methods are used for removing the
heat dissipated by the telecommunications equipment. At moderate
power densities, circulation fans, air-to-air heat exchangers, or
heat pipes may be used. At high power densities, or for
installations where the ambient temperature may exceed the
equipment limit, an air-conditioner may be required. In extremely
cold weather, electric heaters are often required to maintain the
minimum equipment temperature. All of these methods for thermal
conditioning have certain disadvantages or limitations.
Heat exchangers and heat pipes require a relatively large
difference between the ambient temperature and the equipment
cooling air temperature in order to properly transfer heat, thereby
restricting the use of such an approach to cooler ambient
environments. The use of air-conditioners to control temperature
within sealed cabinets also presents difficulties. The heat load
from the equipment in some sealed cabinets is such that the
air-conditioner is required to run even at low ambient conditions,
sometimes as low as 0.degree. F. Some units use variable
speed-condenser blowers and/or a flooded condenser design to reduce
the heat loss through the condenser and allow the air-conditioner
to run at very low temperatures. Other units use a low-ambient vent
to cool the cabinet with outside air when the temperature is too
low to run the air conditioner. These solutions increase the cost
and size of the cooling system. In addition, for small cabinets
designed for locations with severe space limitations, using an air
conditioner is a significant size penalty. Air conditioners are
also a source of noise which may preclude their use in noise
sensitive environments.
SUMMARY OF THE INVENTION
The present invention is a telecommunications equipment cabinet
particularly adapted for use in outdoor locations. The cabinet
comprises an enclosure having an electronics subrack for containing
the telecommunications equipment. The enclosure includes an inlet
opening in a lower portion of the cabinet for drawing cool ambient
air into the cabinet. An exhaust outlet for exhausting heated air
is disposed in an upper portion of the cabinet. In the preferred
embodiment, the cabinet includes a segregated air routing system to
direct air from the inlet, through the subrack, and out of the
cabinet. The segregated air routing system includes a duct that
extends from the air inlet to a subrack plenum disposed directly
below the electronics subrack. An exhaust plenum is disposed
immediately above the subrack and communicates with the exhaust
outlet.
One or more fans draw cool ambient air into the cabinet. The cool
ambient air flows through the inlet to the subrack plenum. A heater
in the inlet duct heats the air at low ambient temperatures. The
air is then pulled through the electronics in the subrack where the
air picks up heat dissipated by the equipment. From the subrack,
the heated air enters the exhaust plenum and is expelled through
the exhaust outlet. The air flow is confined by the duct work in
the cabinet for more effective control of cooling air flow rate and
temperature.
The cooling system includes a thermal controller for monitoring the
temperature within the cabinet and for controlling the operation of
the fan(s) and the heater. The thermal controller is typically a
microprocessor-based controller that receives as input the subrack
inlet air temperature, the subrack exit air temperature, subrack
equipment temperature, and outdoor ambient air temperature. At low
ambient temperatures (low temperature mode), the controller of the
preferred embodiment causes the fans to operate at reduced capacity
to maintain a nearly constant air flow through the subrack. The
power to the heating element is varied to maintain the subrack
inlet temperature constant over a predetermined temperature range
up to a first predetermined temperature. In moderate temperatures,
the power to the heating element is turned off and the air flow is
increased non-linearly as the temperature increases so as to
maintain the temperature at the subrack exit constant. At high
temperatures, the fans operate at full capacity.
The cabinet of the present invention is able to cool the
electronics in compliance with equipment requirements for cooling
air temperature and rate of temperature change without the burdens
of size, complexity, power consumption, and noise of an
air-conditioned cabinet. Protection from air-borne contaminants is
provided by filters disposed in the inlet duct.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the telecommunications cabinet of
the present invention with the enclosure shown in hidden lines.
FIG. 2 is a section view of the telecommunications cabinet.
FIG. 3 is a front elevation view of the telecommunications
cabinet.
FIG. 4 is a block diagram of the control system for the
telecommunication cabinet.
FIG. 5 is a graph illustrating the theoretical and practical
thermal envelopes for the telecommunications cabinet over a range
of ambient temperatures.
FIG. 6 is a graph illustrating the operation of the thermal control
system for the telecommunications cabinet over a range of ambient
temperatures.
FIG. 7 is a graph illustrating the fan operation of one
embodiment.
DETAILED DESCRIPTION
An electronic equipment cabinet 10 includes various electronics
housed within a suitable enclosure 20 having a top 22, a bottom 24,
and sides 26. For purposes of illustration, the electronic
equipment cabinet 10 will be described as housing
telecommunications electronics 34 such as that required for a
cellular telephone base station. Such electronic equipment cabinets
10 typically include both power electronics 32 and communications
electronics 34. The power electronics 32 are typically separated
from the communications electronics 34 by a suitable power
distribution box 30 so as to reduce the hazards associated with
servicing the communications electronics 34. The communications
electronics 34 typically include a variety of communication circuit
modules 42. The circuit modules 42 typically take the form of
removable circuit cards disposed in a vertical orientation and
supported by an electronics subrack 40 within the cabinet enclosure
20. The particular details of the configuration and operation of
the power electronics 32 and communications electronics 34 are well
known to those skilled in the art and are not discussed further
herein except as to assist in understanding the present
invention.
When operating, the communications electronics 34 within the
electronic equipment cabinet 10 should be cooled to help prevent
malfunctions or damage; this is particularly true for electronic
equipment cabinets 10 in locations that are not in temperature
controlled environments, such as outdoor environments. The
electronic equipment cabinet 10 of the present invention includes a
cooling system 50 to cool the communications electronics 34. The
cooling system 50 provides a flow of cooling air across the
communications electronics 34. In general, the airflow is in via
one or more inlet vents 52, through a inlet duct 56 to a subrack
inlet plenum 70, through the electronics subrack 40 to an subrack
exhaust plenum 72, and out one or more exhaust vents 74. At least
one fan 80, and preferably a plurality fans 80, are disposed
downstream from the electronics subrack 40 and pull external air
along this airflow path.
In one embodiment, the inlet vent 52 is disposed along the bottom
of the enclosure 20 and is guarded by a inlet grill 54 sized to
keep out larger nesting and stinging insects and the like.
Equivalently, the inlet vent 52 may be disposed along the sides 26
of the enclosure 20 near the bottom 24. The inlet duct 56 connects
to the inlet vent 52 and includes a lower portion and an upper
portion. The lower portion preferably includes a filter 58 and a
heating chamber 60. The filter 58 helps remove airborne fibers and
further helps control the ingress of insects. The heating chamber
60 includes one or more heating elements 62. The heating element 62
is used to preheat the cooling air entering from the outside when
it is cold so that the air entering the subrack inlet plenum 70 is
above the minimum cooling air temperature specified for the
equipment. Preferably, the power density of the heating element 62
is limited, such as in the range of 6 watts per square inch of
heater surface area or less, to minimize radiant heating of the
electronics subrack 40 and other equipment within the radiation
field of the heating element 62 and to avoid reaching flammability
limits of adjacent components and/or foreign objects entrained in
the airflow. The heating element 62 is preferably an electric
resistance heater and is controlled by the thermal controller 100
as more fully described below. Preferably, the power for the
heating element 62 is regulated by a triac 64 having on/off zero
crossing detection so as to minimize electromagnetic interference
on any associated AC power lines. The upper portion of the inlet
duct 56 connects to the subrack inlet plenum 70. The subrack inlet
plenum 70 is a generally open area, preferably disposed below the
electronics subrack 40, which is designed to distribute the air
being supplied to the electronics subrack 40.
Air from the subrack inlet plenum 70 flows through the electronics
subrack 40 so as to cool the communications electronics 34 therein.
Typically, but not necessarily, the airflow is vertically through
the electronics subrack 40, between the spaced apart circuit cards.
Because the cooling system 50 of the present invention may not be
equipped with humidity control, and to protect from other
contamination, it may be advisable to employ conformal coatings,
protective (but thermally conductive) barriers, sealants, or the
like to keep airborne contaminants and moisture from direct contact
with the communications electronics components.
Air from the electronics subrack 40 exits from the subrack into the
subrack exhaust plenum 72. The subrack exhaust plenum 72 is a
generally open area which collects the airflow from the electronics
subrack 40 and channels it to the fans 80. The fans 80 pull the air
through the electronics subrack 40 and exhaust the same to the
outside through the exhaust vents 74. While only one fan 80 is
required, it is preferred that a plurality of fans 80 be used to
reduce overall size and provide redundancy in case of fan failure.
The capacity and number of fans 80 should vary depending on the
desired capacity of the cooling system 50. Each fan 80 should be
variable speed over a wide range of speeds. Each fan 80 should be
equipped with a backdraft damper to prevent recirculation of
cooling air through any fans
80 that are not in use. To provide a redundant backup, it is
preferred that one fan 80 be held in reserve. In the event that the
optional fan speed sensors indicate that one of the main fans 80 is
not operating properly, such as when the detected speed is 75% or
less than the desired speed, the reserve fan 80 may be substituted
for the malfunctioning fan 80.
A thermal controller 100 controls the operation of the heating
element 62 and the fans 80. The thermal controller 100 includes
suitable control electronics, a plurality of temperature sensors
112,114,116,118, and optional fan speed sensors 120 for each fan
80. The temperature sensors 112,114,116,118 typically include a
pair of subrack inlet temperature sensors 112, one primary and one
backup, a subrack outlet temperature sensor 114, a subrack
equipment temperature sensor 116 and an ambient air temperature
sensor 118. The subrack inlet temperature sensors 112 should be
located in the center of the subrack inlet plenum 70, downstream
from the heating element 62 if present. The subrack outlet
temperature sensor 114 should be located in the center of the
subrack exhaust plenum 72. The subrack equipment temperature sensor
116 should be mounted to a dummy circuit card (not shown) in the
electronic subrack 40 having the same thermal mass as an
operational circuit card. The ambient air temperature sensor 118
should be located on a suitable exterior surface of the enclosure
20, such as in an external cavity near the inlet vent 52.
The thermal controller 100 controls the airflow through the cooling
system 50, and the power of the heating element 62, to maintain the
temperature of the cooling air supplied to the electronics subrack
40 within suitable thermal limits. There are preferably at least
three main operational modes for the thermal controller 100--low
temperature, moderate temperature, and high temperature. The
operational mode is chosen based on the subrack inlet temperature
as indicated by the subrack inlet air temperature sensor 112. For
instance, the low temperature mode may apply when the subrack inlet
temperature is less than 150.degree. C., the moderate temperature
mode may apply when the subrack inlet temperature is between
15.degree. C. and 35.degree. C., and the high temperature mode may
apply when the subrack inlet temperature is above 35.degree. C.
In the low temperature mode, the fans 80 generate a constant
airflow, such as 33% of capacity, while the heating element 62 is
enabled. Preferably the heating element 62 is powered just enough
to maintain the temperature of the air being supplied to the
electronics subrack 40 just above the minimum cooling air
temperature limit. In this mode, the subrack inlet temperature
sensor 112 provides the primary feedback to the thermal controller
100.
In the moderate temperature mode, the heating element 62 is off,
but the fans 80 are enabled. Preferably, the number and speed of
the fans 80 are varied so as to maintain the temperature of the air
exiting the electronics subrack 40 just below the maximum
temperature limit. In order to efficiently control the airflow, it
is preferred that the thermal controller 100 be able to
independently control the speed of each fan 80. In this mode, the
subrack inlet temperature sensor 112 provides the primary input to
the thermal controller 100. The subrack outlet temperature sensor
114 is used to detect excessive temperatures in the subrack exhaust
plenum and to generate an alarm.
In the high temperature mode, the heating element 62 is off, and
the fans 80 operate at full capacity.
In addition, the thermal controller 100 should optionally be able
to operate in a cold start mode. Cold start mode is applicable when
power is first supplied to the communications electronics 34 and
the temperature of the communications electronics 34, as indicated
by the subrack equipment temperature sensor 116, is below the
allowed minimum temperature. In this mode, the communications
electronics 34 is turned off until the electronics subrack 40 can
be preheated to within specification limits. During start-up, the
fans 80 operate to produce a reduced airflow, such as 20%, while
the heating element 62 is engaged. Preferably, the heated air
within the inlet duct 56 is heated by the heating element 62 to a
temperature at or near its maximum allowed temperature, such as
45.degree. C. Heated air from the inlet duct 56 is thereby pulled
through the electronics subrack 40, heating the components within
the electronics subrack 40. Once the electronics subrack 40 reaches
the allowed minimum temperature, the communications electronics 34
are turned on.
The functioning of the present invention may best be described in
the context of cooling communications electronics 34 having
specified maximum and minimum incoming cooling air temperatures,
maximum cooling air temperature rise across the communications
electronics 34, and a maximum cooling air exhaust temperature from
the communications electronics 34. Further, the communications
electronics 34 may have a limitation on the maximum rate of change
in temperature for the incoming cooling air. For purposes of
illustration, the following values will be used:
______________________________________ Incoming cooling air
temperature +5.degree. C. to 45.degree. C. Maximum cooling air
temperature rise 10.degree. C. at maximum air flow Maximum exhaust
temperature 55.degree. C. Maximum rate of temperature change
0.5.degree. C./min. ______________________________________
FIG. 5 shows the allowed theoretical thermal envelope for such a
system as bounded by the maximum exhaust temperature (Tex) and the
minimum incoming cooling air temperature (Tin). This theoretical
thermal envelope is narrowed in practice to allow for system
temperature and control tolerances. The practical thermal envelope
is bounded by the maximum practical exhaust temperature (Texp) and
the minimum practical incoming cooling air temperature (Tinp).
Because it is assumed that the cooling system 50 of the present
invention uses ambient outdoor air to cool the communications
electronics 34, the thermal envelopes are additionally bounded on
their lower side by the actual ambient air temperature (Tamb).
The objective of the thermal controller 100 is to control the
temperature and flow of the air in the subrack inlet plenum 70 and
the subrack exhaust plenum 72 within the practical thermal
envelope, thereby maintaining the temperature of the electronics
subrack 40 within specification limits. In order to do so, the
thermal controller 100 may function as shown in FIG. 6. At low
outside temperatures, such as below 15.degree. C., the thermal
controller 100 operates in low temperature mode and causes the fans
80 to operate at a reduced flow rate of about 33%, and the heating
element 62 is supplied with power according to line "Heater Power".
Note that the heating element 62 may be disabled above 10.degree.
C. for reasons described below. Above 35.degree. C., the thermal
controller 100 operates in high temperature mode, disabling power
to the heating element 62 and running the fans 80 at full capacity.
Between 15.degree. C. and 35.degree. C., the thermal controller 100
operates in moderate temperature mode, disabling the heating
element 62 and varying the flow rate of the fans 80 according to
line "Air Flow". As shown, the airflow rate may vary non-linearly
with temperature in the moderate temperature mode.
In one embodiment utilizing three main fans 80, only selected fans
80 are used at lower flow rates, with other fans 80 being activated
for higher airflow rates. This method of operation is shown in FIG.
7. For instance, in the low temperature mode, two fans 80 are
operated at half speed and the third is off. The reason for using
two fans 80 is that control of fan speed is limited to 50%-100% by
fan manufacturer design constraints. In the moderate temperature
mode, two fans 80 are used with increasing speeds at the lower
airflow rates, and the third fan 80 is activated at the higher
airflow rates. In order to facilitate a smooth airflow transition
from two to three fans 80, the speed of the first two fans 80 is
reduced when the third fan 80 is turned on. After the third fan 80
is activated, all three fans 80 are ramped up in speed if
additional airflow is required. If the ambient temperature begins
dropping, the inverse procedure is used to transition from three
fans 80 to two. Thus, in this embodiment, the airflow through the
electronics subrack 40 does not change dramatically when the number
operating fans 80 changes to provide increased or decreased airflow
through the cooling system 50. The transition from two fans to
three fans occurs at a first pre-determined air flow rate when
demand for air flow is increasing. The transition from three fans
to two fans occurs at a second air flow rate when the demand for
air flow is decreasing. The first air flow rate and second air flow
rate are different (see FIG. 7) to prevent the fans from
jittering.
By varying the operation of the heating element 62 and the fans 80
as shown in FIG. 6, operation may be maintained within the thermal
envelope. In addition, power consumption is substantially reduced.
While the ambient temperature is lower than T.sub.inp, the heating
element 62 is engaged and a small flow of air is induced. However,
the heating element 62 need not be fully engaged; it is only
necessary that the subrack inlet temperature be at or just above
the T.sub.inp. Thus, heater power varies inversely with T.sub.amb
and stops completely when the T.sub.amb is at or above T.sub.inp.
Further, in the moderate temperature mode, the subrack inlet
temperature need only be sufficient to keep the subrack outlet
temperature at or below T.sub.exp. In order to maintain the subrack
exit air temperature below its maximum, air flow will increase as
subrack inlet temperature rises.
In addition, it should be noted that the variation in fan airflow
may need to be higher than that indicated by air flow in FIG. 6
when the comparison of the subrack exhaust temperature sensor 114
and the subrack inlet temperature sensor(s) 112 indicates that the
temperature rise across the electronics subrack 40 is more than the
allowed temperature rise. Further, the heating element 62 power and
the fan 80 airflow may have to vary from the ideal represented in
FIG. 6 to accommodate the restriction on maximum rate of change in
subrack inlet temperature. For instance, the heating element 62 may
be supplied with less power than indicated when the ambient
temperature is rising rapidly in the morning after a cold
night.
The present invention may, of course, be carried out in other
specific ways than those herein set forth without departing from
the spirit and essential characteristics of the invention. The
present embodiments are, therefore, to be considered in all
respects as illustrative and not restrictive, and all changes
coming within the meaning and equivalency range of the appended
claims are intended to be embraced therein.
* * * * *